1. Technical Field
This invention relates to a phosphor thin film used as a light-emitting layer in inorganic EL devices or the like, a method of preparing the same and an EL panel using the same.
2. Background Art
In the recent years, active research works have been made on thin-film EL devices as small-size and large-size, lightweight flat panel displays. A monochromatic thin-film EL display using a phosphor thin film of manganese-doped zinc sulfide capable of emitting yellowish orange light has already become commercially practical as a double insulation structure using thin-film insulating layers 2 and 4 as shown in FIG. 2. In FIG. 2, a predetermined pattern of lower electrodes 5 is formed on a substrate 1, and a first insulating layer 2 is formed on the lower electrode-bearing substrate 1. On the first insulating layer 2, a light-emitting layer 3 and a second insulating layer 4 are successively formed. On the second insulating layer 4, a predetermined pattern of upper electrodes 6 is formed so as to construct a matrix circuit with the lower electrodes 5.
Thin-film EL displays must display images in color in order that they find use as computer, TV and similar monitors. Thin-film EL displays using sulfide phosphor thin films are fully reliable and resistant to environment, but at present regarded unsuitable as color displays because EL phosphors required to emit light in the primaries of red, green and blue have poor characteristics. Engineers continued research on SrS:Ce (using SrS as a matrix material and Ce as a luminescent center) and ZnS:Tm as a candidate for the blue light-emitting phosphor, ZnS:Sm and CaS:Eu as a candidate for the red light-emitting phosphor, and ZnS:Tb and CaS:Ce as a candidate for the green light-emitting phosphor.
These phosphor thin films capable of emitting light in the primaries of red, green and blue suffer from problems of emission luminance, emission efficiency and color purity. Thus color EL panels have not reached the commercial stage. Referring to the blue color among others, a relatively high luminance is achieved using SrS:Ce. However, its luminance is still short as the blue color for full color display and its chromaticity is shifted toward green. There is a desire to have a better blue light-emitting layer.
To solve these problems, thiogallate and thioaluminate base blue phosphors such as SrGa2S4:Ce, CaGa2S4:Ce, and BaAl2S4:Eu were developed as described in JP-A 7-122364, JP-A 8-134440, Shingaku Technical Report, EID 98–113, pp. 19–24, and Jpn. J. Appl. Phys., Vol. 38 (1999), pp. L1291–1292. These thiogallate base phosphors are satisfactory in color purity, but suffer from a low luminance and especially, difficulty to form a thin film of uniform composition because of the multi-component composition. It is believed that thin films of quality are not obtainable because of poor crystallinity resulting from inconvenient composition control, formation of defects resulting from sulfur removal, and admittance of impurities; and these factors lead to a failure to increase the luminance. In particular, thioaluminate base phosphors are quite difficult to control their composition.
In order to develop practical full color EL panels, processes of preparing phosphor thin films for blue, green and red light in a consistent manner and at a low cost are necessary. Since matrix materials and luminescent center materials of phosphor thin films individually have differing chemical or physical properties as described above, the preparation method differs depending on the identity of the phosphor thin film. Then, if film depositing conditions are set such that a phosphor thin film of a specific composition may produce a high luminance, a phosphor thin film for another color fails to develop a high luminance. This necessitates a plurality of film forming apparatus for the overall process of manufacturing a full color EL panel. The manufacturing process becomes very complex, and the cost of panel manufacture increases.
Moreover, the EL spectra of the aforementioned blue, green and red EL phosphor thin films are all broad. When they are used in a full color EL panel, RGB necessary as the panel must be cut out of the EL spectra of the EL phosphor thin films, using filters. Use of filters complicates the manufacture process and, still worse, brings about a lowering of luminance. When RGB is taken out through filters, the luminance of blue, green and red EL phosphor thin films is lost by 10 to 50% or more so that the luminance of the panel is reduced below the practically acceptable level.
In order that an EL panel perform on a practical level, it is required to maintain a luminance over a long period of time, that is, have a long luminance life.
To solve the above-discussed problems, there remains a need for red, green and blue phosphor thin films capable of emitting light at a high luminance and a satisfactory color purity enough to eliminate a need for filters, and having a long luminance life. It is also required that such red, green and blue phosphor thin films be prepared by an identical film-forming method or film-forming apparatus.